Coatings are applied to light transmissive or reflective members in order to impart desired optical properties to the members. For example, one or more coatings may be applied to a screen of a computer monitor in order to provide polarization or to reduce reflection. One method for reducing light reflection from a substrate, such as a cathode ray tube (CRT), is to coat the surface with an antireflective layer having a thickness of approximately one-quarter wavelength of the light to be reflected. The antireflective layer may be an organic material, such as a polymer, or an inorganic material, such as a metal fluoride, where the deposited layer has a refractive index that is less than that of the CRT. A high reduction in reflection is achieved when the refractive index of the deposited antireflective layer equals the square root of the refractive index of the CRT material. This approach has limitations. Single layer, low refractive index antireflective coatings often exhibit distinct colorations, particularly regarding reflection.
Antireflective coatings having wider bandwidths, and consequently less coloration, may be obtained by using multiple deposited layers. Two common antireflective coating designs are the quarter-quarter (QQ) and the quarter-half-quarter (QHQ) stacks. That is, the antireflective coating is formed of a number of layers having differing refractive indices, each equal in optical thickness to one-quarter (Q) or one-half (H) of a wavelength sought to be antireflected.
A detailed description of an antireflection stack is contained in U.S. Pat. No. 5,744,227 to Bright et al., which is assigned to the assignee of the present invention. At least two pairs of layers are formed to provide the antireflection properties. One layer of each layer pair is an electrically conductive, light transmissive, inorganic material having an index of refraction in the range of 1.88 to 2.15, while the other layer is a light transmissive inorganic oxide having an index of refraction in the range of 1.4 to 1.6. The stack performs well for achieving the desired optical properties.
Antireflective coatings are typically fabricated with non-absorbing material. Consequently, the reduction in reflection is accomplished by increasing the energy that is transmitted into the member for which antireflection is a concern (e.g., the CRT). Some absorption along the light path may be beneficial to enhancing image contrast. To this end, absorbing materials may be added to the transparent material (glass) that is used in fabricating the CRT. Alternatively, the antireflective coating that protects the CRT may include one or more absorbing layers. The absorbing layers may have dispersion in n and k which leads to good antireflection properties (i.e., low reflection and wide bandwidth), with relatively few layers. See for example, U.S. Pat. No. 5,691,044 to Oyama et al., where titanium nitride (TiN.sub.x) is shown to be a preferred absorbing layer. However, simple stacks involving only two layers (e.g., TiN.sub.x and SiO.sub.2) generally provide good antireflection properties only for a narrow range of transmission values. The use of metal nitrides as absorbing layers is also taught in U.S. Pat. No. 5,091,244 to Bjornard. In some cases, nitrides are not the preferred materials for antireflective stacks, since the optical properties of the nitrides are often very dependent on the deposition conditions (e.g., the relative amount of N.sub.2 and O.sub.2 in one deposition environment). However, further improvements in optical characteristics are sought.
U.S. Pat. No. 4,846,551 to Rancourt et al. describes an optical filter assembly to enhance image contrast and reduce glare of a CRT. A number of layers are formed on a substrate to achieve the desired optical properties. The first layer is an aluminum oxide layer having a thickness of at least 170 nm, which is approximately 3/8 wave of optical thickness at a design wavelength of approximately 500 nm. In the preferred embodiment, atop the aluminum oxide layer are alternating layers of nickel and magnesium fluoride. The first nickel layer is formed on the aluminum oxide. The aluminum oxide is intended to enable the nickel layer to be securely joined to the substrate. The nickel layer has a thickness of about 13 to 80 angstroms. The next layer is a magnesium fluoride film having a thickness equivalent to a quarter wave optical thickness. A second nickel layer is formed to have a thickness in the order of 75 angstroms. Finally, a second magnesium fluoride layer has a quarter wave optical thickness. The patent notes that test measurements of this optical filter coating in use with a CRT evidence that only 0.5-0.8 percent of the integrated reflected light reached the eyes of a viewer of the display and that the integrated light transmittance of the image display was approximately 84 percent. In comparison, 10-12 percent of the reflected light reached the eyes of the viewer when the optical filter coating was not used.
While available optical coatings achieve acceptable results, what is needed is an antireflective coating that provides contrast enhancement and that is relatively easily process-tailored to achieve a visible light transmissivity (Tvis) within a wide range of values, while maintaining good antireflection properties. Furthermore, it is desired that absorbing layers be relatively simple to deposit, and thus result in high manufacturing productivity.